The present invention relates to a fiber lasers and, more particularly, to rare-earth element-doped fiber lasers operating at the three-level transition, and to fiber optic structures for fiber lasers.
Optical communication networks demand high transmission speeds, wide bandwidth, and great channel capacity. Another important requirement is for reliable signal transmission at an appropriate optical power level for good signal detection after tens of kilometers of signal travel. Optical signals transmitted in a network are typically optically amplified after each 10 to 15 kilometers of transmission path. The amplification is performed by optical amplifiers, which represent a single or double clad fiber structure with the fiber core doped by rare earth elements. The optical signals carrying information travel through the fiber core. In order to perform the amplification, high power optical pumping radiation is also introduced into the fiber core by direct coupling to it or through the cladding. The pumping radiation raises the energy levels of the doping to enable amplification of the signal through stimulated emission. It is noted that the amplifiers are separate from the fibers used for signal transmission. Such amplifiers are made of fibers approximately ten to fifteen meters in length.
In conventional single-mode fibers, the wavelength of minimum loss is around 1.5 micrometers. The ability to amplify this wavelength is therefore extremely important in fiber optic networks. Erbium Doped Fiber Amplifiers (EDFA), when operated in the so-called xe2x80x9cthree-level modexe2x80x9d and pumped at a wavelength of approximately 980 nanometers, have the capability of amplifying signals of the required 1.5-micrometer wavelength. For efficient optical amplification, EDFA""s in turn require high power single-mode coherent pumping at a wavelength close to 980 nanometers.
Ytterbium (Yb) doped fiber lasers are sources of high power, high brightness, single mode coherent approximately 980-nanometer optical radiation, provided they are operated at a three-level transition and not at their more-easily attained four-level transition. Operation of an Yb doped fiber laser at a three-level scheme presents a number of problems. One of the problems is the significant absorption loss of the three-level emission peak at about 980 nanometers. To overcome, these absorption losses and lase at approximately 980 nanometers the Yb doped fiber must be xe2x80x9cbleachedxe2x80x9d; that is, more than 50% of all Yb ions must be excited to the upper excitation state.
Although various attempts have been made to produce Yb lasers operating at a three-level transition, there are no cost-effective, high-power, single transverse-mode, Ytterbium pump sources currently-available. Generally, three-level operation in an Ytterbium fiber laser is achieved in two ways:
a) by increasing the efficiency of the coupling of pumping radiation into the cladding; and
b) by improving absorption of the coupled pumping radiation into the Yb-doped core.
Increased coupling of the pumping radiation to the cladding is attained primarily by using air-clad fibers with a high numerical aperture (NA). Improving the absorption of the coupled pumping radiation by the doped fiber core is typically achieved by a fiber geometry that encourages the optical path of the pumping energy to cross the fiber core as much as possible. Additional pumping energy is absorbed by the core each time such a crossing occurs.
U.S. Pat. No. 4,815,079 to Snitzer et al. (herein denoted as xe2x80x9cSnitzerxe2x80x9d), which is incorporated by reference for all purposes as if set forth fully herein, discloses a clad pumped fiber laser where the improvement of absorption of the coupled pumping radiation by doped fiber core was obtained by placing the fiber core off-center relative to the cladding. FIG. 1 illustrates this geometry. A fiber core 20 is surrounded by a first inner cladding 22, a second inner cladding 24, and an outer cladding 26. The reasoning behind the off-center placement is that when pumping is characterized by multi-mode operation, it is well-known in the art that the radial distribution of the pumping energy in inner cladding 22 is such that a large part of the pumping energy is located away from the center of inner cladding 22, and that to maximize the absorption of pumping energy, fiber core 20 should also be located away from the center. It is noted that the term xe2x80x9cinner claddingxe2x80x9d herein denotes any cladding that is interior to the outer cladding of a fiber. A fiber may have more than one inner cladding, as is illustrated in FIG. 1 with a first inner cladding 22 and a second inner cladding 24. The term xe2x80x9couter claddingxe2x80x9d herein denotes a cladding whose outer surface is the exterior surface of the fiber. A fiber can have at most one outer cladding, as is illustrated in FIG. 1 with outer cladding 26.
Although the prior-art structure illustrated in FIG. 1 is efficient in coupling pumping radiation into the core, an off-center fiber structure is not practical since optical transmission lines and networks typically have a coaxial structure, and connecting coaxial transmission lines to an off-center amplifier such as that of the prior art (FIG. 1) is extremely difficult.
Improving absorption of the pumping radiation in the Yb-doped fiber core is also achieved by increasing the number of places the pumping energy""s optical path crosses the fiber core. U.S. Pat. No. 5,533,163 to Muendel (herein denoted as xe2x80x9cMuendelxe2x80x9d), which is incorporated by reference for all purposes as if set forth fully herein, teaches use of cladding having non-cylindrical shapes. FIG. 2 shows such a geometry. A cylindrical single-mode fiber core 30 is surrounded by an inner cladding 32 in the form of a non-rectangular, convex polygon so that the propagating pump energy is induced to form an essentially uniform radiation field in which the various radiation modes comprising the pump energy are isotropically distributed. A cylindrical outer cladding 34 presents an overall cylindrical shape externally. A variety of additional cladding shapes, some of which are shown in FIGS. 3A, 3B and 3C, are also disclosed by Muendel. Muendel teaches criteria for proper convex polygon selection and generally states that good results may are obtained by use of any k-sided convex polygon that satisfies the condition:
xcex8=360xc2x0/k xe2x80x83xe2x80x83(1) 
where xcex8 is the central angle and k≳3.
Muendel discloses that fibers with off-center structure and irregular polygons are especially advantageous. Fabrication of a fiber optic structure in accordance with the invention is accomplished by machining a preform to the desired cross section and then drawing the preform according to techniques known in the art. Such preparation of a preform and machining of multiple facets on the preform, however, are operations that undesirably complicate fiber fabrication.
The term xe2x80x9ccylindricalxe2x80x9d herein denotes any surface describable as the normal (perpendicular) locus of a circle along an axis. As used herein, such an xe2x80x9caxisxe2x80x9d need not be a straight line segment, and may even be a closed curve. Thus, as the term xe2x80x9ccylindricalxe2x80x9d is used herein, a drawn fiber optic structure may have a cylindrical external surface even when deformed or bent so as not to correspond to the mathematically developable surface of a right cylinder of rotation. The term xe2x80x9ccoaxialxe2x80x9d as used herein denotes a relationship between two or more cylindrical surfaces having the same axis and corresponding to loci of circles of different diameters. The cross-section of such a cylindrical surface normal to the axis is a circle.
U.S. Pat. No. 6,031,849 to Ball et al. (herein denoted as xe2x80x9cBallxe2x80x9d), which is incorporated by reference for all purposes as if set forth fully herein, discloses a double-clad Yb fiber laser operating at a three-level transition at approximately 980 nanometers, along with a method of manufacturing the fiber. In order to enhance the pumping energy coupling efficiency, the form of the laser""s inner cladding matches the geometry of the pump source, which is the emitting area of a powerful single stripe, broad area diode. The rectangular form of the cladding improves core absorption of the pumping energy. Fabrication of such a fiber laser, however, is complicated because the fiber core has a round form whereas the inner cladding is rectangular. The preform rod should be slot ground with equal and opposing slots around one axis of the inner core. This, too, is a complicated operation.
U.S. Pat. No. 6,157,763 to Grubb et al., which is incorporated by reference for all purposes as if set forth fully herein, discloses a double-clad optical fiber that has an inner cladding with a cross-sectional shape that is non-circular, but which maintains a good end-coupling profile. The cross-sectional shape of the inner cladding is such that two perpendicular distances across the shape, each of which passes through a geometric center of a core of the fiber, are equal for all angular positions. The shape of the preferred embodiments is such that the flat surfaces of the cladding are at right angles to each other along the outside boundary of the shape, and are created by abrading away the surface of a cylindrical inner cladding of a glass preform of the fiber. Additional surface abrading operations complicate fiber production, so that this approach is difficult.
U.S. Pat. No. 5,907,652 to DiGiovanni, et al., which is incorporated by reference for all purposes as if set forth fully herein, discloses a method of coupling pumping energy from low brightness sources (such as diode arrays) into the inner cladding of a double-clad fiber, taking advantage of the inner cladding""s large cross-sectional area and high numerical aperture (NA). As the multi-mode pump light crosses the core, it is absorbed by the rare-earth dopant. The geometry presented in DiGiovanni is coaxial, although it is also said to be possible to increase the overlap of the pump light with the core by making the inner cladding elliptical. The high numerical aperture of the inner cladding is achieved by using an air cladding having a low effective refractive index.
Despite all of the above-described prior-art efforts and the need, there are no commercially available, cost-effective Ytterbium fibers lasers operating at the desired three-level transition. There is thus a widely-recognized need for, and it would be highly advantageous to have, an efficient and cost-effective cladding-pumped Ytterbium-doped fiber laser operating reliably at a three-level transition. In particular, there is a need for an Yb-doped fiber laser that can be directly pumped by regular laser diodes, provide high power single mode output at approximately 980 nanometers, and maintain high slope efficiency. This goal is met by the present invention.
An object of the present invention is to provide an efficient and cost-effective cladding-pumped rare-earth element-doped fiber laser operating reliably at a three-level transition and operating at high power single mode output in the range of approximately 980 nanometers.
It is a further object of the present invention to provide a high numerical aperture air-clad fiber structure enabling easy coupling with optical radiation pumping sources.
It is still a further object of the present invention to provide an air-clad fiber supporting cylindrical fiber geometry and enhancing the efficient utilization of pumping energy and absorption within a cylindrical coaxial fiber core.
It is yet an additional object of the present invention to provide a method of cost-effective manufacturing of an air-clad fiber structure that enhances the efficient mode conversion process within the fiber core and inner cladding.
These and other objectives of the invention are attained by providing a fiber optic structure with a geometry that features a cylindrical fiber core which is coaxial with a cylindrical outermost cladding, and where the fiber core is surrounded by at least one eccentric inner cladding. The terms xe2x80x9ceccentricxe2x80x9d and xe2x80x9ceccentricityxe2x80x9d herein denote any geometric and/or optical properties for a cladding which does not exhibit full rotational symmetry, including, but not limited to: forms whose mathematical centers of mass are radially displaced from the axis of the cylindrical outermost cladding; forms whose geometrical or optical properties vary according to the azimuthal angle with respect to the fiber axis; forms whose geometrical or optical properties have a rotational aperiodicity with respect to the fiber (that is, relative to a polar coordinate system associated with the fiber, or a cross-section thereof); and forms which are non-cylindrical. The present invention provides a number of embodiments of eccentricity in such fiber optic structures, including air-claddings with variably-sized and/or variably-placed capillaries, as well as regions of altered refractive index. Because the cladding is eccentric and/or irregularly-shaped, a fiber laser according to the present invention benefits from improved coupling of the pumping energy field""s modal distribution pattern within this eccentric and/or irregularly-shaped cladding into the core. But because the core is cylindrical and coaxial, signals into and out of the core are easily coupled out of and into standard coaxial transmission fibers. Thus, a fiber optic structure according to the present invention overcomes prior-art limitations. It is also noted that the principles of the present invention are applicable in cases involving fiber doping by rare-earth elements in general. For convenience in discussion, the present invention is described in terms of the non-limiting case of Ytterbium doping. The present invention is also useful for fiber lasers doped with other rare-earth elements.
Ytterbium-doped fiber laser operation at the three-level transition is enabled by efficient mode conversion that increases the absorption of pump radiation by the Ytterbium-doped fiber core. In embodiments of the present invention utilizing air cladding, the interface between the inner cladding and the air cladding or air channels is not a smooth one. Air holes influence the interface so that it becomes xe2x80x9cflower-likexe2x80x9d in some cases, or asymmetric and irregular in other designs. This interface stimulates mode conversion from inefficient ring modes propagating at the periphery (and located far from the fiber core) to modes that have a high overlap with the Ytterbium-doped fiber core. Pumping power absorption is thereby increased to a level that surpasses the Yb bleaching threshold, enabling efficient operation at the three level transition.
A fiber optic structure according to the present invention can be used with an optical energy pumping source and with an Ytterbium doped core, to provide an optical fiber laser. For example, an optical fiber laser system according to the present invention that operates at approximately 980 nanometers may be pumped by one or more solid state laser diodes. Normally, such a fiber would have a number of claddings in addition to the eccentric and/or irregularly-shaped cladding a described above, and there would also be means for coupling the optical pumping source into the fiber. The core emits stimulated radiation of the desired wavelengths according to the excitation provided by the optical pumping.
In accordance with present invention a fiber optic structure may have at least one of the claddings as an air cladding between the innermost cladding and the outermost cladding, where the air cladding is formed and sized so as to be eccentric with respect to the outer cylindrical cladding. Moreover, such air cladding can be of hollow capillaries of various diameters, whose refractive index is selected according to the inequality ncore greater than ninner cladding greater than ncapillary greater than nouter cladding. Furthermore, the fiber may have an inner cladding of photosensitive material which has been UV-treated to have regions with different refractive indices. Such UV-treating of photosensitive material to produce regions of altered refractive index is well-known in the art, and is commonly used, for example, to produce reflective Bragg gratings. According to the present invention, UV treatment also contributes to more efficient mode conversion.